Direct comparison of novel unidirectional sandwich coupon designs for accurate tensile failure strain determination of carbon fibre epoxy material
Topic(s) : Experimental techniques
Co-authors :
Gergely CZÉL (HUNGARY)Abstract :
Carbon fibre/epoxy (CF/EP) is increasingly preferred for high-performance applications offering outstanding lightweighting potential e.g. for the transportation industry. Despite the long success of CF/EP there is still space for increasing the accuracy of their material property determination. Tensile testing of UD CF/EP is of ultimate interest for the whole supply chain of high-performance composite products. The common practice is to focus on the uniaxial tensile (and sometimes compression) properties and estimate the rest based on the measured properties and the available technical literature. Parametric studies run with finite element (FE) simulations confirmed that this approach is feasible.
Tensile testing of UD CF/EP sounds simple, but there are a number of challenges to overcome, most of which concerns the gripping of specimens. Dumbbell specimens having wide gripped sections and narrow gauge section cannot be applied as longitudinal splitting disables the extra material in the gripped section. Current standards recommend prismatic specimens with end-tabs bonded to the gripped sections to prevent damage from the serrated gripping elements. However, the use of end-tabs generates significant stress concentrations around their edge due to the change of cross-section which usually leads to premature failure there.
Hybrid composite coupons with continuous protective layers made by co-curing UD S-glass/epoxy (GF/EP) and CF/EP prepreg plies were proposed recently by the author et al [1] and generated consistent gauge-section failures, due to the elimination of stress-concentration confirmed by FE simulation. Other approaches like butterfly-shaped coupons [2] and arrow-shaped end-tabs on prismatic coupons showed promising results when compared in [3]. Two main areas for improvement were identified to make the continuous protection approach considerable for standardisation: i) The CF/EP layer exhibited transverse direction undulations in the cross-section. ii) The strains measured on the surface of the sandwich specimens had to be corrected for thermal residual strain.
This study directly compares alternative layer integration techniques using the same materials: i) curing the protective layer onto a solid CF/EP test plate and ii) bonding three cured plates together in a sandwich construction, to address the outstanding challenges. Manufacturing processes were developed with special focus on the challenging bonding procedure for the new sandwich coupons. Gripping conditions were optimised by applying different grit-size sandpaper tabs to the ends of the coupons (see figure 1). Double camera 3D video-extensometer and electronic strain gauges were utilised for ultimate measurement accuracy. Thermal residual strains were considered, where relevant, when the average failure strains of the different coupon types were compared (see figure 2). The bonded coupons were found suitable for further development.
Tensile testing of UD CF/EP sounds simple, but there are a number of challenges to overcome, most of which concerns the gripping of specimens. Dumbbell specimens having wide gripped sections and narrow gauge section cannot be applied as longitudinal splitting disables the extra material in the gripped section. Current standards recommend prismatic specimens with end-tabs bonded to the gripped sections to prevent damage from the serrated gripping elements. However, the use of end-tabs generates significant stress concentrations around their edge due to the change of cross-section which usually leads to premature failure there.
Hybrid composite coupons with continuous protective layers made by co-curing UD S-glass/epoxy (GF/EP) and CF/EP prepreg plies were proposed recently by the author et al [1] and generated consistent gauge-section failures, due to the elimination of stress-concentration confirmed by FE simulation. Other approaches like butterfly-shaped coupons [2] and arrow-shaped end-tabs on prismatic coupons showed promising results when compared in [3]. Two main areas for improvement were identified to make the continuous protection approach considerable for standardisation: i) The CF/EP layer exhibited transverse direction undulations in the cross-section. ii) The strains measured on the surface of the sandwich specimens had to be corrected for thermal residual strain.
This study directly compares alternative layer integration techniques using the same materials: i) curing the protective layer onto a solid CF/EP test plate and ii) bonding three cured plates together in a sandwich construction, to address the outstanding challenges. Manufacturing processes were developed with special focus on the challenging bonding procedure for the new sandwich coupons. Gripping conditions were optimised by applying different grit-size sandpaper tabs to the ends of the coupons (see figure 1). Double camera 3D video-extensometer and electronic strain gauges were utilised for ultimate measurement accuracy. Thermal residual strains were considered, where relevant, when the average failure strains of the different coupon types were compared (see figure 2). The bonded coupons were found suitable for further development.